A gas sensor with a heated gas-sensitive semiconductor as a measuring element is disclosed in DE 38 10 409 A1. The measuring element is embedded according to this document directly in a gas-permeable, heat-insulating cover in order to reduce the release of heat and to reduce the energy consumption in this manner. In addition, the sensor as a whole is said to be stabilized and made largely insensitive to mechanical effects such as shocks.
An essential drawback of this arrangement is the poorer diffusion of the gas to be measured compared to the prior-art arrangements with support wires and catalytic measuring elements arranged thereon, i.e., especially heat tone sensors called Pellistors. The cover of the measuring elements represents an additional diffusion barrier. The drawbacks become obvious especially in case of measuring high-boiling gases such as nonane or xylene. This is due both to the response time and the sensitivity of the correspondingly covered gas sensors being markedly poorer than without a cover.
Another gas sensor with poor accessibility of the gas to be measured to the catalytic measuring elements is described in U.S. Pat. No. 5,601,693.